Pulse modulation amplifiers are commonly used to supply drive current to inductive loads, such as linear, voice-coil, and DC motors, using pulse width modulation (PWM). A pulse modulation amplifier, such as a PWM amplifier, receives an analog waveform and outputs a square wave pulse. The square wave pulse has an amplitude and duration such that the integrated energy of the pulse is equivalent to the energy of the sampled input analog waveform multiplied by a gain factor created by the amplifier.
The resulting PWM waveform may be filtered to produce an analog waveform replicating the original input waveform multiplied by the gain factor. Typically, the inherent electrical or mechanical frequency response of the controlled system, such as an electric motor, performs the filter function.
Where a filter circuit is used in a controlled system, a basic inductor capacitor (LC) filter or a Cauer-Chebyshev (C-C) filter circuit may be used. FIG. 14 shows a conventional C-C filter 10 using a current source 11, two inductors 12 and 14 in series between the voltage input V.sub.in and output V.sub.out. A third inductor 16 is connected to ground and between inductors 12 and 14. A capacitor 18 is disposed between inductor 16 and ground, while a second capacitor 20 and a load resistor 22 connect output V.sub.out to ground. However, conventional filters such as a LC filter or a C-C filter, as shown in FIG. 14, have undesirably high resonance peaks at low frequencies. FIG. 15 is a plot showing the peak resonance characteristics of C-C filter 10 relative to frequency. As shown in FIG. 15, there is an approximate 8.5 dB peak resonance at 10 KHz. Commonly, an increase of resistance in series with the inductor of an LC filter, may be used to lower the peak resonance, however, that decreases efficiency and dissipates heat.
In a conventional motor drive system, motor control is generally considered non-critical and thus a smooth drive current is not a concern. Further, currently available motor drive systems are unconcerned with total harmonic distortion (THD) characteristics, which cause noise and generate excessive undershoot or overshoot ringing effects. Where a controlled system requires a high degree of precision, however, the amplifier system must be highly linear. For instance, photolithographic systems require high resolution when in the scanning mode. Further, power transfer efficiency is important in systems that require generation of large acceleration and deceleration forces, for instance in a stepping mode to rapidly change positions of the system. Thus, a high performance motion control system requires a high degree of linearity in scanning mode and high power transfer efficiency when in stepping mode.
Although analog amplifier systems typically are more linear, less noisy, and exhibit less distortion than equivalent PWM amplifier systems, analog amplifier systems suffer from poor power transfer efficiency, which creates heat. Conventional PWM amplifier systems, on the other hand, do not provide drive current in a linear fashion and typically have poor THD characteristics. Thus, conventional PWM amplifier systems are inadequate to produce the high degree of precision and power transfer efficiency required by current high performance motion control systems.